The present invention relates to an amplifier circuit, and more particularly, to an amplifier circuit the output stage of which is comprised of a complementary push-pull circuit and which can be suitably used as an operational amplifier.
In a circuit including an operational amplifier, for example, an integrated circuit for processing analog signals, it is necessary that the operational amplifier should provide an output having as great a voltage amplitude as possible, in order to process the signals with a high S/N ratio. Most preferably, the maximum output voltage of the operational amplifier should reach the power supply voltage. It is necessary for the amplifier to produce an output of a great voltage amplitude, particularly when the power supply for the amplifier is a small-power, low-voltage battery. Further, when it is required that the operational amplifier consume as little power as possible, the output-stage transistor of the amplifier, whose bias current is large, must assume an operation similar to a so-called "class B operation", so that the amplifier consumes a minimal amount of power during a non-input period. This is one of the design requirements when the power available to the amplifier is limited.
In order to satisfy these requirements, the output stage of the operational amplifier must be:
(1) a common-source circuit of a voltage-gain type whose output is taken from the drain of an output FET; or PA0 (2) a common-emitter circuit of a voltage-gain type whose output is taken from the collector of a bipolar transistor.
Let us use, herein, the word "transistor" in its broadest sense, encompassing both an FET and a bipolar transistor. Also, let us use the word "first electrode" to mean the source of an FET and also the emitter of a bipolar transistor, which receive carriers; the word "second electrode" will mean the drain of the FET and the collector of the bipolar transistor, which supply the carriers, and the word "third electrode" will mean the gate of the FET and also the base of the bipolar transistor, which controls the flow of the carrier. Then, it can be said that the output-stage circuit of the operational amplifier must have its first electrode connected to the ground, in order to fulfill the requirements described above. This is because, since the potential of the third electrode of the transistor cannot become higher than the power supply voltage, the maximum output voltage would be lower than the voltage of the third electrode by the threshold voltage of the transistor if the output is taken from the first electrode of the transistor, whereby the maximum output-voltage amplitude will substantially decrease. When the transistor is a silicon semiconductor element, its threshold voltage is about 1 V for an FET, and about 0.6 V for a bipolar transistor. The loss of the output-voltage amplitude, which results from the threshold voltage of the transistor, cannot be negligibly small when the power supply voltage is, for example, 3 V (two 1.5 V dry cells).
An amplifier circuit whose output stage is a common-source, complementary push-pull circuit using FETs is disclosed in R. Gregorian et al. IEEE JSSC., SC-14, 6, pp. 970-980, December 1979. More precisely, this circuit is a CMOS operational amplifier. The main components of this CMOS operational amplifier are shown in FIG. 1. As is shown in this figure, a differential pair of NMOS (N-channel MOS) FETs Q1 and Q2 is provided. A current mirror circuit comprised of a pair of PMOS (P-channel MOS) FETs Q3 and Q4 is coupled to the loads of NMOSFETs Q1 and Q2. The differential pair of NMOSFETs Q1 and Q2 and the current mirror circuit constitute a differential amplifier, or voltage-amplifying stage VA which receives an input signal. The output signal of voltage-amplifying stage VA is level-shifted by source-follower stage SF formed of NMOSFETs Q5 and Q7. Voltage-amplifying stage VA further comprises NMOSFET Q6. This NMOSFET Q6 functions as a constant-current source. Similarly, NMOSFET Q7 of source follower stage SF functions as a constant-current source. The signal, which has been level-shifted by source follower circuit SF, is supplied to the gate of NMOSFET Q8. The output voltage of voltage-amplifying stage VA is applied to the gate of PMOSFET Q9. NMOSFET Q8 and PMOSFET Q9 constitute output stage OS which is a common-source, complementary push-pull circuit.
Since output stage OS of the amplifier circuit is a common-source, complementary push-pull circuit, the maximum positive and negative output voltages can be equal to the positive and negative power supply voltages +E and -E, respectively. In other words, the amplifier can provide a great output amplitude. Further, the two FETs forming output stage OS, which performs a push-pull operation, can operate in the class AB mode, provided that source-follower stage SF achieves an appropriate levelshifting of the output signal of voltage-amplifying stage VA. Hence, the amplifier circuit of FIG. 1 can satisfy the requirements described above.
The power-supply voltage applied to the operational amplifier shown in FIG. 1 is not always constant. Therefore, it is required that the amplifier should function correctly even if the power-supply voltage varies within a certain range. In particular, when the operational amplifier is made in the form of an integrated circuit, the elements forming the amplifier are inevitably somewhat different in their characteristics, due to minor variations resulting during the manufacturing process. It is necessary for the amplifier to function correctly in spite of this characteristic difference among its elements.
In the amplifier circuit of FIG. 1, both voltage-amplifying stage VA and source-follower stage SF are driven by a constant current. Hence, the bias current does not vary when the power-supply voltage changes. However, the output voltages of stages VA and SF vary when the power-supply voltage changes. Consequently, the bias current of output stage OS changes in response to the changes of the power-supply voltage. To make matters worse, the bias current of output stage OS varies even if the characteristics of the elements, for example, the threshold voltages of FETs Q8 and Q9, fluctuate. In the worst case, both FETs constituting output stage OS are cut off due to an undesirable combination of a power-supply voltage and a threshold voltage of either FET. When both FETs of stage OS are cut off, the amplifier circuit will inevitably have a dead-band. Or, due to such an undesirable combination, an excessive bias current will flow through output stage OS, in some cases.
The conventional amplifier circuit, whose output stage is a complementary push-pull circuit comprising an NMOSFET and a PMOSFET and which supplies an output from the node of the second electrodes of these MOSFETs, can indeed provide a great output amplitude from a limited power-supply voltage and can reduce power consumption. Nonetheless, it has the inherent drawback that the bias current of the output stage varies when the power-supply voltage fluctuates or when the MOSFETs have different characteristics.
Therefore, to use this amplifier circuit practically, it is necessary to restrict fluctuations in the power-supply voltage to a narrow range. In addition, to manufacture this amplifier circuit, it is necessary to use elements which differ as little as possible in their characteristics. As a result, it is difficult to manufacture the amplifier circuit with a good yield, and the cost will be unavoidably high. cl SUMMARY OF THE INVENTION
It is accordingly the object of the present invention to provide an amplifier circuit which can provide a great output amplitude from a relatively low powersupply voltage, and can operate stably, without being adversely influenced by the fluctuation of the powersupply voltage or by the difference in characteristic among the elements forming the circuit and/or by the fluctuation of the characteristic of each element.
An amplifier circuit according to the invention comprises a voltage-amplifying stage, an output stage including a push-pull circuit comprised of at least one complementary pair of output transistors, and a drive stage for driving the output transistors in accordance with the output of the voltage-amplifying stage. The drive stage includes a subtraction unit and a reference voltage-generating unit. The reference voltage-generating unit generates a reference voltage. The subtraction unit subtracts the output voltage of the voltage-amplifying stage, which is based on a first potential of a power supply, from the reference voltage, thereby outputting a difference between the reference voltage and the output voltage of the voltage-amplifying stage. The output voltage of the subtraction unit is converted to a voltage signal based on the second potential of the power supply. This signal drives one of the output transistors of the push-pull circuit, whereas the output voltage of the voltage-amplifying stage drives the other output transistor.
The bias current of the output transistor driven by the output voltage of the voltage-amplifying stage is determined by the constant-current source for the differential amplifier constituting the voltage-amplifying stage. This bias current is maintained even if the power-supply voltage changes. The voltage signal, which is supplied to the control electrode of the other output transistor, is based on the second pole-potential of the power supply. Hence, the bias current of this output transistor is also maintained even if the power-supply voltage changes.
Both output transistors of the output stage can constitute a class AB amplifier, and can perform a push-pull operation, since they are of common-second-electrode configuration. The amplifier circuit of the present invention can, therefore, provide an output whose amplitude is substantially equal to the power-supply voltage, and can reduce power consumption.
The present invention can provide an amplifier circuit which has an output stage made of a class AB, complementary push-pull circuit, and receiving a bias current remaining constant even if the power-supply voltage fluctuates, or even if its elements have different characteristics. The amplifier circuit can, therefore, provide an output whose amplitude is nearly equal to the power-supply voltage, and can reduce power consumption. Moreover, the amplifier circuit of the invention has a sufficient versatility since its power-supply voltage is not as limited as a conventional one. Further, it can be made of transistors whose characteristics differ to some extent, provided that a proper conductivity type is selected for each transistor. This helps to improve the manufacturing yield of the amplifier circuit, particularly when the circuit is